IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION 1 A Survey on Cooperative Diversity for Wireless Networks F. Gómez-Cuba, R. Asorey-Cacheda and F.J. González-Castaño
Abstract—Diversity, i.e. transmitting multiple replicas of a signal, may mitigate fading in wireless networks. Among other diversity techniques, the space diversity of multi-antenna systems is particularly interesting since it can complement other forms of diversity. The recent cooperative diversity paradigm brings the advantages of multi-antenna space diversity to single antenna networked devices, which, through cooperation and antenna sharing, form virtual antenna arrays. However, cooperative H diversity is a complex technique and research on this topic is still in its early stages. This paper aims at providing a general survey on the theoretical framework; and the physical and medium access control proposals in the literature. Index Terms—Wireless networks, cooperative diversity, MAC protocols. S D
I. INTRODUCTION Due to time-variant fading, the attenuation in a wireless channel may vary due to multiple circumstances. Thus, wire- less system designs typically include some degree of diversity so as to provide the receiver with several realizations of the signal, which increases the chances of a successful transmis- sion [1]. Fig. 1. With cooperation, more nodes are subject to interference. In this case, Many forms of diversity are possible depending on how the helper (H) that receives the transmission of the source (S) and forwards it to the destination (D), generates extra interference that would not be present different available channels or subchannels replicate the signal. if direct transmission was used. Time diversity consists of transmitting replicas with enough separation in time to allow signal decorrelation. Frequency diversity relies on multiple carriers, and space diversity sys- better reception in each receiver along the path and, thus, tems have multiple antennae that are sufficiently spaced and improvements in range, rate or autonomy. These tradeoffs are receive the same information [1]. analyzed in [1] and [3], which conclude that the strategy is Wireless user devices tend to be constrained in size, com- profitable. For this reason, future wireless network designs plexity and power, rendering previous diversity methods un- should consider cooperation capabilities. feasible. To cope with this problem, the cooperative diversity Cooperative diversity, as a technique to combat fading, paradigm has appeared recently. In it, single-antenna net- should find its niche in the upcoming generations of mo- worked nodes coordinate themselves to form a virtual antenna bile data networks, typically cellular architectures, granting array, seeking the advantages of MIMO spatial diversity [1]: a higher throughput by means of spectrum reutilization. Con- each source associates itself with other nodes, acting as helpers sequently, the cooperative scenario requires new analyses as that first receive the transmission of the source and then the introduction of third parties tends to increase interference relay the information. As a result, one extra transmission is [4], [5] (see Figure 1). needed to send the information to the receiver and the number Although mobile cellular networks are the natural target of hops in each route is doubled. The increase in cost is of cooperative diversity techniques, any wireless network only due to the second stage, since the broadcast nature of affected by fading can use them. Since the benefit increases the wireless network allows simultaneous transmission to as as more potential helpers conform the network, dense sensor many helpers as needed in the first stage. Furthermore, multi- networks [6], [7] represent a good application scenario for hop transmission [2] with cooperative diversity may favor a low complexity cooperative techniques. A cooperative virtual backbone may also be of interest in ad-hoc networks [8], [2]. Manuscript received 22 March 2011; revised 26 July 2011. F. Gómez-Cuba is with Gradiant, CITEXVI, Campus, 36310 Vigo, Spain. Cooperation is a versatile strategy, which can be exploited R. Asorey-Cacheda and F.J. González-Castaño are with the Departamento de for purposes other than diversity. Cetinkaya and Orsun pro- Enxeñaría Telemática, Universidade de Vigo, ETSI Telecomunicación, Cam- posed a MAC protocol in which nodes cooperate to adapt their pus, 36310 Vigo, Spain (e-mail: [email protected]; [email protected]; [email protected]). contention windows, improving fairness [6]. Nevertheless, Digital Object Identifier 10.1109/SURV.2011.082611.00047 cooperative diversity differs from other cooperation techniques 1553-877X/11/$25.00 c 2011 IEEE 2 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
Phase-I S tx. S tx. S Phase-II D 1 2 2 2 T T 2 2
Fig. 3. TDMA medium sharing without cooperation. The assignment period T is divided in two medium accesses.
S1 tx. S2 relays S1 S2 tx. S1 relays S2
T T T T S D 4 4 4 4 1 1 Fig. 4. TDMA medium sharing with cooperation.The assignment period T is divided in four slots, encompassing two direct transmissions and two relay transmissions. The same information is transmitted twice and the rate is halved. Fig. 2. Wireless network example: Two sources S1 and S2 access the medium alternately to transmit to the two destinations D1 and D2; both nodes of each communication pair overhear the transmission of the other It is possible -by halving the transmission rate- to allow pair. a node to allocate half its transmission time to its own information and the other half to relaying information (figure in the sense that the improvement opportunities rely on the 4). physical layer (PHY). Consequently a cross-layer design is Using the above forwarding policy, the overall outage O(P 2) required if other layers are involved. probability would be of the order of o , because a packet The rest of this paper is structured in four parts. Section II would only get lost in the case of outage in two independent reviews the background from the perspective of information routes (one composed of two links, with the helper in be- theory, emphasizing the fact that a point-to-point approach tween). Evidently, this model is oversimplified: link outages cannot achieve the full capacity of a network [9]. Different are assumed to be independent, the PHY layer processes theoretical transmission models are reviewed and compared. individual packets independently, the helper must fully (and Section III analyzes recent PHY architectures. Relying on successfully) decode the packets it retransmits, the destination information theory, we discuss a representative group of decodes each replica independently, and at least one of the cooperative PHY techniques. Section IV surveys cooperative replicas must be successfully decoded at the destination. However, if we analyze the problem from the perspective MAC layers. Finally, section V concludes the paper. of information theory as in [10], the system entangles much more mutual information than that defined by the hop-by- II. INFORMATION THEORETICAL APPROACH hop success-requiring characterization. Mutual information is A. Philosophy given by the combination of the direct link and the two-hop Let us consider a four-node wireless network with two link from a source to a destination. This means that the desti- transmitters S1 and S2 and two destinations D1 and D2 as nation can use information from both the direct transmission shown in figure 2. The two transmitters share the medium and the relayed transmission in order to decode the data. by some means, such as, for instance, pre-assigned periodic Laneman et al [11] considered two classical relay algorithms symmetric TDMA slots (figure 3). Assuming a Rayleigh depending on whether or not the relay/helper decoded the fading coefficient α that is slowly varying and flat, the link received signal, (decode-and-forward (DF) and amplify-and- outage probability P0 is defined as the probability that the forward (AF), respectively). In cooperative AF, an optimum signal-to-noise ratio seen by the receiver (|α|2SNR,where receiver accesses the information of two parallel noisy chan- SNR is the signal-to-noise ratio at the transmitter) is lower nels, one of which has a classical AF relay. Regarding DF, than the minimum needed, i.e. the rate is above the mutual either the source-to-relay or the relay/source-to-destination information of the channel [10]: links limit the maximum achievable rates. R − P P R> |α|2SNR P |α|2 < 2 1 o = log2(1 + ) = SNR B. Theoretical System Models (1) Based on classical relay techniques, Laneman et al suggest where R is the binary rate per Hertz, defined per slot. some improvements [10]: It is remarkable that, due to the broadcast nature of the • Selection Relaying uses either AF or DF and, if the relay wireless medium, a node that is idle at a certain moment reception signal to noise ratio (SNR) is low, the relay can overhear the transmissions of its peers. In typical wireless stays silent, allowing the source to retransmit instead. systems these receptions are simply discarded. However, since • Incremental Relaying isbasedontheassumptionofsome different nodes experience independent realizations of the kind of acknowledgement. So, regular communication fading phenomena, the success probabilities are independent takes place first and, if no acknowledgement (ACK) is and, thus, a transmitter can rely on a helper to create a form received, the relay sends its copy of the signal to the of diversity. destination. GOMEZ-CUBA et al.: A SURVEY ON COOPERATIVE DIVERSITY FOR WIRELESS NETWORKS 3
d(R ) norm Ch1 S1 tx. S2 relays S2 S3 relays S2 SN relays S2 Ch S tx. S relays S S relays S S relays S 2 DF 2 2 1 2 3 2 N 2 Ch S tx. S relays S S relays S S relays S Direct 3 3 1 3 2 3 N 3 AF Incremental AF ChN SN tx. S1 relays SN S2 relays SN SN−1 relays SN
T T T T 1 N N N N
Fig. 6. TDMA-based system with diversity order N and Rnorm =1/N . In a given user channel, all other users relay information in different time slots.
Ch1 S1 tx. (S1) relays S1 0.5 1 R norm Ch2 S2 tx. (S2) relays S2
Ch3 S3 tx. (S3) relays S3 Fig. 5. Diversity for direct transmission, AF, DF and incremental AF. Incremental AF achieves the upper bound of a 2-antenna system. ChN SN tx. (SN ) relays SN
T T The metric of interest is the outage probability Po(SNRnorm) 2 2 as a function of transmitter SNR values, normalized by the N R =1/2 minimum SNR required by each protocol (the normalization Fig. 7. STC-based system with diversity order and norm .Ina given user channel, all other users may relay information simultaneously. The allows a fair comparison by separating the improvement due operator D(Si) stands for the subset of relays that receive correctly from Si to diversity and the improvement due to the different spectral and actually perform the relaying. efficiencies of each protocol). The diversity order of a system is defined as the exponent that asymptotically relates SNR increase to Po decrease. This efficiency, which is known as the diversity versus multiplexing was formulated in [12] as: trade-off [14], which is explained in section III-A. In [15], the authors proposed dropping the orthogonality log(Po) assumption on source and relay nodes, in what they called d ≡− lim (2) SNR→∞ log(SNR) the Non-Orthogonal-AF Protocol (NAF), allowing the source to join the transmission in the second slot. For DF, they A system with Nt transmitter antennae and Nr receiver proposed the Dynamic DF Protocol (DDF) without fixed antennae is said to provide full diversity when d = Nt × Nr. In addition, the trade-off between diversity and normalized time slots, in which, assuming incremental redundancy codes, spectral efficiency -i.e. R normalized by its maximum sus- the relaying phase starts once there is enough information tainable value- can be analyzed: full diversity is achievable to decode (hence the term dynamic). Additionally, multi- user centralized schemas were outlined by analyzing Multiple when Rnorm is zero as shown in figure 5. The normalization expression is: Access Channel (NAF up-link) and Multi-User Broadcast Channel (DDF down-link) architectures. R R Contemporarily, Nabar, Bölcskei and Kneubühler designed norm := σ2 SNR (3) log2(1 + s,d ) space-time codes with three orthogonality options (table I) 2 [16].They arrived at the same conclusion: allowing the source where σ is the channel variance. It may be identified as the s,d to transmit during the second phase (Mode 1) increases overall improvement in R related to SNR, called multiplexing gain in performance. This is compatible with AF and DF. [12]: R The work in [12] extended that in [15], including the results r ≡ lim (4) SNR→∞ log(SNR) of Nabar, Bölcskei and Kneubühler [16], whose policy-1-AF (NBK-AF) is equivalent to NAF. It achieves the upper perfor- In [13], Laneman and Wornell studied systems with several mance bound for the AF family, surpassing the orthogonal AF helpers. Two cooperative schemas were proposed: of Laneman, Tse and Wornell [10] (LTW-AF). The proposed 1) Each user channel is divided into N time slots, being N DDF was compared to LTW-DF. It outperforms LTW-DF and, the number of transmitter (figure 6). At each time slot in almost all cases, NBK-DF (figure 8). In addition, DDF a different relay transmits, thus dividing R by N. outperforms the bound of the AF family (NAF). 2) The user channels only have two slots (figure 7). All In [17], the authors focused on the differences between relays transmit simultaneously using space-time coding static and dynamic architectures. They provided two new in the second slot. schemas, Extended Static DF protocol (ESDF), allowing a More generally, all the surrounding nodes can potentially fixed cooperation in time division, and Extended Dynamic DF contribute to increasing the receiver information and those (EDDF), which is dynamic. ESDF is superior to NAF and information sources can be combined in a constructive way by other static protocols, and EDDF is a modification of DDF some means. This diversity increase has some cost in spectral with a slightly better diversity d. 4 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
TABLE I d(r) ORTHOGONALITY OPTIONS FOR SOURCE AND RELAY TRANSMISSIONS. Phase Option 1 Option 2 Option 3 2 Ideal I S→H,D S→H,D S→H NAF II S+H→D H→D S+H→D 1.8 LTW-DF 1.6 NBK-DF C. Conclusions and Open Issues 1.4 DDF Cooperative diversity brings the advantages of multi- 1.2 antenna transmission techniques to single antenna nodes within a network. Despite the fact that it is necessary to split 1 resources to allocate relaying transmissions, this technique can still offer important benefits. The study of cooperative 0.8 diversity is similar to that of classical MIMO, using diversity 0.6 gain and multiplexing gain as metrics, and taking into account that multiplexing gain is restricted to values below 1. 0.4 There are multiple options from information theory to im- 0.2 prove cooperative diversity transmission systems. Considering the basic approach of a single helper that relays source r information, the first option is to increase the number of 0.2 0.4 0.6 0.8 1 helpers. In addition, the restriction of orthogonal transmission may be removed using coding techniques borrowed from Fig. 8. Diversity of NAF, LTW-DF, NBK-DF and DDF. Note that DDF is multi-antenna transmitter design. Thus, it is possible to allow the best option, and NBK-DF is slightly better in a small region. several helpers to transmit simultaneously, or the source to transmit fresh information while previous information is being relayed. Time-division planning represents another degree of • Power saving: by transmitting with lower power, or freedom to enhance cooperative diversity, either by associating equivalently with lower SNR, relying on cooperation to SNR
d(r) 1 Conv. Code Ideal 4 1 M + 1 Repetition coding 2 CC Non cooperative Source Space Time Coding Opportunistic Relaying 1 CCc 2 Relay
1 / 1 Fig. 10. 2 4 distributed convolutional code. The bits in black (from the 1 complementary 2 convolutional code) are meant to be transmitted by the helper.
1 D. Cooperation through Network Coding r In this type of coding, which was originally proposed for 1 0.5 1 wired networks, the nodes forward linear combinations of M+1 the messages they receive. This schema differs from chan- Fig. 9. The diversity curves of OR and STC are equivalent. This means that nel coding in that it operates at packet level. For example, −N both outage probabilities tend asymptotically to SNR when SNR tends supposing that a destination receives three coded packets in SNR to infinity, but for low their behavior may still differ. three transmission slots x, y and x ⊕ y, for a given slot 2 error rate ρ, the third packet turns Psuccess =(1− ρ) into 2 Psuccess =[1+2ρ](1 − ρ) [22]. Other MAC-PHY proposals that also select the helpers are 1) Binary Linear Combination: Xiao et al [23] criticized based on medium access contention depending on Channel the cost in energy or time of cooperative relaying, proposing State Information (CSI). Nevertheless, this has the disadvan- instead a binary XOR function to build a combined message tage that, unless all potential helpers lie within mutual range, and allocating full channel resources to its transmission. Even some means to avoid collision are necessary. though the authors remark that theirs is a network coding A solution, it is very similar to code cooperation: being iL(t) A and iR(t) the information vectors and GL and GR the coding C. Code Cooperation A A matrices, then the XOR operation iL (t)GL ⊕ iR(t)GR might Most classical systems include some kind of forward error A A GL be expressed as a code concatenation [iL (t)iR(t)] . correction codes at the PHY layer, and many of them are GR systematic, which means that each codeword begins with Thus, the authors proposed an iterative decoder combining the original message bits followed by some redundancy. The the data in both streams to recover the information. analogy between systematic FEC and cooperative diversity is 2) Non-Binary Finite Field Linear Combination: The pro- noticeable: in both cases additional information is appended posal presented in [24] and extended in [25] departs from to the original message. The comparison between cooperative the assumption that there exist direct and orthogonal relay diversity and repetition coding pointed out in [10] leads to channels (like in TDMA cooperative diversity), and focuses what is formally known as code cooperation. on the composition of the relay messages. For example, in a Srefanov and Erkip [20] elaborated on the idea of a more four time-slot system, the binary coding schema [23] would sophisticate coding architecture shared by the source and the result in the four messages [x1|x2 ⊕ x1|x2|x1 ⊕ x2],which relay. In the proposed schema, the source message is protected outperform the system in [10]. However, this code is still by a 1/4 convolutional code that is carefully selected so that suboptimal, because, if packets 1 and 3 get lost, no information the first two bits exactly match a 1/2 convolutional code can be recovered. (figure 10). The source punctures its transmitted message, Moreover, the authors show that the binary field is insuf- sending only the first two bits. The helper is expected to ficient to achieve the full potential of cooperation, and it 1 GF m receive the message correctly with a 2 Viterbi decoder due is suggested employing Galois fields ( (2 )) instead. As to its proximity to the source, and finally the complementary shown in figure 11, by using GF(4), the two relay components code-bits of the word are computed and transmitted. If the are now different and decoding is successful for any two relay failed to decode the message, in the absence of a messages. carrier, the source would start transmitting the remaining code- In [24], direct and relay phases alternate, while in [25] bits by itself. Moreover, the destination should store these, coding is extended to super-frames, consisting of multiple 1 merge the two symbol streams and perform full 4 Viterbi direct and relay slots (figure 12). By establishing an analogy decoding. Diversity emerges from the fact that, the higher the with the maximum distance separation in block coding, the Es average energy per symbol σ2 , the better the Viterbi decoder solution is found in well-known Reed-Solomon codes. performs; therefore providing half of the symbols through an 3) Superposition Modulation: Larsson and Vojcic [26] independent channel mitigates fading losses [21]. proposed energy sharing by simultaneous transmission using 6 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
Phase-I Phase-II Phase-I Phase-III H Phase-II S2 Phase-IV x1 2x2 ×1 ×Nt x2 x2 D S1 D x1 ×Nt x1 Fig. 13. A Nt-antenna source using a cooperative single antenna relay N +1 x1 x2 provides the destination with t realizations of the message. S1 antennae as diversity. In brief, a block coding model consists of mapping a L-symbol vector x onto an L × N matrix G(x) Fig. 11. GF(22) network code for a 4-slot TDMA. Each source composes whose columns correspond to the signal to be transmitted by redundancy packets by combining its message and the received message (x1 N x each of the antennae. Thus the discrete equivalent channel and 2). Both original messages can be recovered by receiving any two y G x h w packets could be written by blocks as = ( ) + Many methods have been proposed in the literature by Frame 1 Frame 2 Frame 3 tuning different system parameters such as delay, gain, code, S2 S2 S2 x2[n] x2[n + 1] x2[n + 2] antenna, etc. The following is an overview to clarify the
x2[n] x2[n + 1] x2[n + 2] protocols in section IV (the reader may refer to [29] -Section x [n] D x [n + 1] D x [n + 2] D 1 1 1 I.A Related Work- for a deeper description, to [30] for an x1[n] x1[n + 1] x1[n + 2] approach to CSI-feedback relay phase and gain adaptation S1 S1 S1 based on MIMO beam-forming, and to [31] for asynchronous code design). Frame 4 Frame 5 S2 S2 1) Classical Space-Time Coding: Anghel et al [32] studied 1,2 2,2 space-time coding for classical multi-antenna systems and D D showed that it performs adequately in a distributed virtual
1,1 2,1 array system. They assumed that the relays know the code, S1 S1 and that each relay utilizes a unique column/antenna. Laneman and Wornell [13] elaborated on deserter helpers Fig. 12. GF-based 6/10 network code for a 5-slot super-frame. In the (selected potential helpers that do not cooperate). In an Or- first three frames the sources send six messages in total. Then, four different thogonal Space Time Block Code (OSTBC), removing a helper redundancy packets are computed and transmitted within the remaining two is equivalent to shortening the code by the corresponding frames. column, and since the columns of the matrix are orthogonal, the shortened matrix still defines an orthogonal space-time superimposed signals. The main idea is to send the discrete code. Nevertheless, a column subset from an N-OSTBC is signal x[n]= 1 − γ2x(f)[n]+γx(r)[n],inwhichγ2 < 0.5 is not usually the optimum (N − 1)-OSTBC. Cheng et al [33] the energy-sharing factor. Although this is not a classical finite analyzed an OSTBC base station with a helper relay (figure field operation, it is definitely an algebraic linear combination 13). They proved a diversity order of Nt +1 (where Nt is and therefore it corresponds to network coding. The concept the number of antennae of the transmitter), thus showing that of composed constellation allows the signal to be considered cooperative and local space diversities may benefit mutually. as a single digital transmission. 2) Issues Regarding Distributed Space-Time Coding: Some 4) Complex Field Network Coding (CFNC): Building on authors have reviewed STC use in cooperative diversity. Some the principle of PHY layer network coding, Wang and Gian- common study areas are: GF m nakis [27] proposed substituting (2 ) with the classical • STC design techniques constrained by available helpers C complex field approach of symbol constellations. The and cooperation systems. sources can transmit simultaneously because their signals are • Asynchronous helper tolerance: in classical MIMO archi- separated in the complex plane, and the relay is modeled tectures, all signals/antennae are synchronized. as a separate entity. Additionally, according to [28], the • Notification of the code to the helpers. CFNC signal can be enhanced with dirty paper coding,which • Behavior in case of helper desertion. allows the effects of interference to be minimized when the • Opportunistic exploitation of arriving nodes (which were transmitter is aware of them. not present when the virtual array was formed). • Exploitation of very large helper sets H,forwhich E. Cooperation through Space-Time Coding OSTBCs are unfeasible. The STC transmission technique for multi-antenna systems These problems cannot be solved separately, as there are mentioned in section III-A exploits the signals in separate trade-offs between them. GOMEZ-CUBA et al.: A SURVEY ON COOPERATIVE DIVERSITY FOR WIRELESS NETWORKS 7
STC Linear Combiner F. Conclusions and Open Issues It is possible to achieve in practice the benefits predicted Receiver Channel Encoder by information theory at the physical layer. Some techniques are completely novel, while others are based on preexistent MIMO technologies, or they are borrowed from other areas c of knowledge. Some of them focus on simplicity (best relay, binary linear combination), but most of them rely on rather Fig. 14. Architecture of a STC random combination relay. After reception ( x) and codification (G( x)), the whole STC signal for all N antennae is complex coding strategies. computed and the actual antenna is fed with a linear combination of them all The relations between the many appealing proposals for the s = G( x) c ( i i). PHY layer are not clear: as there are known bounds on the gains, the superposition of two proposals does not necessarily convey the sum of their benefits. Hybrid systems and PHY layer comparisons must be taken into account. In simultaneous 3) Cooperative STCs with Antenna Selection: An initial transmissions such as CFNC or STC, the problem of imperfect approach would be the assignation of one specific antenna oscillator synchronization must be addressed. of the code to each relay. Nevertheless a single helper trans- In the Best-Relay approach some helpers may not need mitting the wrong signal may degrade the overall code, and to participate. Code Cooperation shows that the relationship consequently DF and CRCs are employed to ensure that only between the two messages may be more elaborate than mere the helpers that have received a good message collaborate. replication. Network Coding solutions show that some extra This means that the actual relay set is an aleatory subset D(H) benefit may be obtained if the relays include their own of the set of helpers H. In addition, the helper set itself may information, although the MAC layer becomes more complex. be random as long as any node with successful decoding is Finally, Space-Time Coding is the way classic MIMO systems allowed to relay without further restrictions. transmit simultaneous replicas of the same signal. There is To avoid notifications, random antenna selection has also apparently no reason for these techniques to be mutually been proposed. Although it is an elegant solution, the diversity exclusive. order is the average of the possible results in a multinomial It is also important to stress that some proposals depend N ,N N N distribution ( a h) (where a and h represent the number directly on current technologies and therefore so their lifespans of antennae in the code and the number of helpers, respec- do. tively). Under-exploitation of resources is likely to occur. 4) Cooperative STCs with Virtual Antenna Linear Combi- IV. MAC LAYER PROPOSALS y Sh w nation: By rewriting the equivalent channel = + , A. General Description and Necessity with S = G(x), each column si corresponds to the signal transmitted by a relay. Then, the symbols transmitted by each Both the telecommunications operators and the end-users relay can be modified by rewriting S = G(x) × C,where would reject a wireless network with cooperative diversity if Na×N the PHY layer required manual configuration. Therefore, the C ∈M h is a combination matrix that builds the trans- mission of the relays from the rows of G(x). With this change role of the MAC layer is essential. In addition to cooperation the relays and the code antennae are decoupled. Furthermore, control, the MAC layer must support error recovery, dynamic optimization and mobility support. if the i-th row of C has several non-zero elements, the i- th relay transmits a linear combination of signals of the code As far as traditional MAC design is concerned, PHY is a antennae (implemented as in figure 14). This model allows the lower-level service that can be requested upon will. However, gain tunning and/or antenna selection by simple adjustments in cooperative diversity MAC (CDMAC), this is an over- in C. Summing up, any arbitrary combination of antennae at simplification since the behavior of the cooperative diversity the relay is possible, yielding the same properties as any local PHY (CDPHY) must be finely tuned. As a result, cross-layer STC of choice except for a controllable degradation of the design is a must, in which the CDPHY serves the CDMAC channel y = G(x)h + w, h = Ch. with communication capabilities and the CDMAC serves the CDPHY with advanced planning capabilities. Sirkeci-Mergen and Scaglione analyzed error probability and C matrix design criteria to achieve full diversity order [29], and showed that C matrix design criteria are equivalent B. Examples in the Literature to classic full-rank criteria for space-time block codes applied Next we introduce different protocol proposals from diverse to G(x)C. They also formulated the maximum achievable authors. We refer the reader to tables III to VII to compare diversity order min(Nh,Na). Yiu et al proposed that each them. Rather than discussing the particularities of each work node should have a signature vector ci to define its unique in depth, in section IV-C we describe the MAC design problem combination of matrix columns [34]. Random C matrices for cooperative diversity in general. were proposed in [29] and [35], such that no signaling at 1) CMAC: Chou and Ghosh proposed CMAC in [36]. It all is required. Moreover, Sirkeci-Mergen and Scaglione also seeks the integration of cooperative diversity with extended considered different random distributions [29] and together IEEE 802.11g wireless local area networks. They designed with Sharp they extended the system to asynchronous nodes a MAC layer signaling that coordinates the sources and the in [35]. helpers. They also proposed an extension of the protocol for 8 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
Phase-I Direct Path Phase-II Relaying H H H Phase-III 1 2
RRTS1 RRTS2(SNRS→R) S I D
RRTS Fig. 16. Cooperation in each link of a multi-hop route: The source (S) or 1 intermediate node (I) transmission is overheard and stored by its respective helper, which forwards the transmission to the next intermediate node or the S D destination (D).
RCTS TABLE II DIFFERENT COOPMAC VERSIONS. Fig. 15. DCF-based triangular handshake. From the first Relay Request To Send (RRTS) packet, the helper and the destination can measure the channels Ref. Network Coop. Tech. Signaling that separate them from the source. The relay piggybacks the measurement [45] WiFi Best Relay DCF-based on a second RRTS packet, which also allows the destination to measure the [45] WiFi Best Relay DCF-compatible H-D channel and notify the optimal strategy to the source in the Relay Clear [4] WiFi Best Relay DCF-based To Send (RCTS) packet. [4] WiFi Source combining DCF-based [47] WiFi Source combining DCF-based [48] WiFi RSTBC DCF-based multiple relays named FCMAC, based on data separation in [49] WiFi RSTBC DCF-based Modified 802.16-j [52] WiMAX RSTBC blocks, which are separately protected with Reed-Solomon OFDMA FEC and relayed by different helpers. [50] WiFi RSTBC DCF-based 2) C-MAC: The schema in [37], as others below, is based [51] WiFi RSTBC DCF-based on the Distributed Coordination Function (DCF) of the IEEE 802.11 standard [7]. Performance criteria aim at energy saving, featuring CDMA transmissions. It supports routing in coop- 8) Phoenix: This protocol integrates network coding in erative diversity environments, elaborating on the idea that cooperative CSMA networks [44]. For this purpose, a reactive directional information may be obtained from the received model called CCSMA was developed, and network coding is signals. allowed in the relays. Coordination messages are based on 3) Relay-enabled DCF (rDCF): ZhuandCao[38]devel- DCF with a three-case based handshake process to determine oped a triangular handshake mechanism (Figure 15) based on what messages the destination has previously stored for re- DCF to coordinate communications between the source, the verting the network code. helper and the destination. From the result of the handshake, 9) CoopMAC family: This refers to a series of related MAC the receiver chooses the cooperation and the rates for each layer proposals [45], [4], [8], [46], [47], [48], [49], [50], [51] hop as in other multi-rate proposals [39]. for cooperative diversity, based on IEEE 802.11 DCF and an 4) Opportunistic Relaying (OR): Noting the excessive adaptation from IEEE 802.16 [52]. All proposals are similar, complexity of STC, this technique is designed as a single- changing one feature at a time. The proposals are enumerated helper system supported by relay selection [19]. Full diversity in table II. The source performs a mapping of its surroundings, has been analytically demonstrated. According to that analysis, and AMC allows rate adaptation (see figure 17). The criterion the best helper is selected thanks to a DCF-based protocol in for route selection is total transmission time: which medium contention decision is initialized with the CSI. L Tdirect = τd + 5) Power Aware Relay Selection (PARS): Chen et al [40] Rsd studied power-aware relay selection strategies. By modifying L L OR criteria, PARS selects relays using an Optimal Power Tcoop = τc + + Rsr Rrd Allocation (OPA) algorithm. 6) CD-MAC: Moh et al [41] seeked to improve link re- In these equations, Tx is the overall transmission time in liability, rather than range or rate as previously mentioned mode x (the minimum of the modes is chosen), τx is the protocols. In their approach, also based on DCF, the hops initialization time, L is the packet length and Rx is the in the direct route are protected by DSTC (Distributed STC) transmission rate of each hop. cooperative backup links, which are only activated when 10) Multi Hop Aware Cooperative Relaying (MHA-Coop- the signal is weak. In multi-hop transmissions, each step is Relaying): Adam et al recall that simple substitution of direct independently backed by cooperation (figure 16). links with pairs of cooperative links would alternate SIMO and 7) Cooperative Triple Busy Tone Multiple Access MISO steps (figure 16), instead of providing the full route with (CTBTMA): In [42], Shan et al used carrier sensing for MIMO transmissions [53]. As a solution, two kinds of helpers notification. The messages are based on DCF with the are identified: those that are only suitable for one DSTC step addition of busy tone medium reservation inspired by [43], and those that can cooperate in two consecutive steps. Relay and Helper Busy Tones (HBT) are used to select the best selection gives a bonus to double helpers, which allows a helper by contention. routing schema like that in figure 18. GOMEZ-CUBA et al.: A SURVEY ON COOPERATIVE DIVERSITY FOR WIRELESS NETWORKS 9
TABLE III TABLE OF STUDIED PROTOCOLS IN TERMS OF THE NEIGHBORHOOD MAPPING SERVICE.
Protocol Neighborhood Mapping CMAC Not necessary FCMAC Not necessary Passive listening in helpers, active distribution of rDCF willing lists Regular transmission of “hello” packets and estima- C-MAC tion of angular position OR Not necessary Rmin R2 R... Rm−1 RMax S R > 2Rmin PARS Not necessary H CD-MAC Passive listening at the source CTBTMA Not necessary Phoenix Not necessary R R > 2R coopMAC-I Passive listening at the source min min coopMAC-II Passive listening at the source c-coopMAC Passive listening at the source RcoopMAC Passive listening at the source D Passive listening at the source with optional pilot coopMAX signals for measurement MHA-CR Passive listening at the source Passive listening at the source with pending ACK fairMAC counter Fig. 17. AMC rate concentric ranges and 2-hop exploitation: since the helper DC-MAC Not necessary node is closer to the source it can receive the same packet much faster and forward it again also at a high rate; hence if the two rates are greater than 2Rmin the end-to-end rate is better than the direct one. of link qualities and cooperation possibilities. There are many Direct Path approaches, ranging from completely passive ones (such as Relaying H H Second relaying 1 2 plain neighbor discovery when transmissions are sensed) to completely active ones (such as polling the surroundings until all neighbors are detected). There also exist hybrid approaches that seek a compromise between polling overhead and missing S I D neighbors. Table III lists the approaches in the protocols we have reviewed. In addition, it is necessary to discover the tables of the Fig. 18. MHA enhancement of all links in a multi-hop route. In addition to cooperating with a particular hop in the path, the helpers that are close to neighbors in order to design the cooperative strategies, and their intermediate nodes may keep enhancing the following hops along the again trade-offs emerge. For example, in [38], the helper route. creates and distributes a willing list of the source-destination pairs that it considers it may enhance, and in [46] the source merely listens. Unfortunately, even if a node is sensed as being 11) Distributed Cooperative MAC for Multi-hop Wireless able to cooperate, it might refuse to do so. The proposal Networks (DCMAC): Shan et al [5] proposed a slightly in [46] assumes optimistically that all known neighbors are modified version of OR to support multi-rate by modifying collaborative and takes their lack of relaying as packet losses. the contention mechanism with shortened timers when AMC Instead, cooperation willingness may be squeezed from the en- rate adaptation is available. vironment using announcement, credit systems, game theory, 12) FairMAC: Bocherer and Mathar [54] expressed their etc. concern about the energy cost of cooperation since there is Finally, there is concern about mapping precision [49] a trade-off between energy per transmitted bit and achieved because cooperative diversity is devised to combat time variant throughput. Their proposal, FairMAC, allows the selection of fading, and network reachability is random. Bearing this in the desired cooperation factor α ∈ (0, 1), which represents the mind, neighbor table entries must be discarded as they get old. limit of packets to be relayed for each own packet transmitted. Furthermore, even the neighbor discovery mechanism itself Packet counters and a dual ACK mechanism to notify the needs to track the network changes: if fading is too variant source are used. or the nodes are too bursty, passive hearing will not suffice, whereas if everything is slow and smooth, frequent polling C. Services Required for Cooperation would result in unnecessary overhead. All proposed solutions have their benefits and drawbacks, 2) Helper Set Design: In a network map, the cooperation and none of them is completely superior to the others. In this possibilities of the neighbors (i.e. potential helpers) are known section, we identify the common design areas and the different individually. As there may be several options for cooperation, approaches that the protocols follow in each area. the information on the helpers must be processed into a unique 1) Neighborhood Mapping: Most cooperative MAC proto- best option for each destination. However, the structure and cols require an image of their surroundings, typically imple- size of the best helper set depends heavily on the performance mented through a neighbor table, possibly featuring estimates criteria and the PHY layer. In this regard, best-relay systems 10 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
H H H H H H H s i i i d S1 D3 H D1 S D S Ii Ii Ii D
H H S2 D2 Fig. 20. An ideal example of cooperative multi-hop route with diversity order d =2in all steps. Each node is aided by a helper, and intermediate nodes use them both for distributed reception and transmission. Phase I: Helper group formation Direct Path I Cooperative enhancement H H I S1 D3 I D H S I D1 I I S D I I
H H Fig. 21. Cooperative enhancement of multi-hop routes. Each intermediate S2 D2 node in the path (or the source itself) may be assisted by helpers (not depicted). An I node knows the full list of steps and it is allowed to replace direct forwarding with cooperative transmission towards farther I nodes (or Phase II: Nt × Nr MIMO even the final destination) in order to shorten the path.
H H 1 S1 D3 H network routing combines multi-hop and cooperative MIMO . D1 Consequently, the longer the distance in hops towards the S D destination, the more set-to-set hops need to be considered (Figure 20). H H In general, multi-hop switching for ad-hoc networks with S2 D2 cooperative diversity is an open research field [2]. The exam- ple in [55] is very illustrative but it relies on an excessively Phase III: Data collection regular mesh topology. In [59] and [53], classical routing is used, and the hops are enhanced a posteriori (figure 21). Fig. 19. A three-hop cooperative system with two helper sets. First, As initial approaches they are very good, but they do not both the source and the destination contact their own groups of helpers to perform distributed transmission/reception. Then, a full MIMO transmission guarantee the optimality of their solution. is performed, and finally the destination collects data from the distributed 3) Cooperation Analysis and Decision: It is necessary to reception group. compare the non-cooperative scenario with the cooperative options in terms of proficiency and cost. The difference with helper set design is that, instead of assuming cooperation and optimizing the helpers, the cooperation mechanism itself is selected. Optimal helper sets may be given a priori to refine the should order helpers by expected performance, while systems analysis criteria, or they may be designed a posteriori just for with any form of signal combination should simultaneously the chosen technique (except for the case of direct transmis- consider the combination policy and the set of signals in order sion). Table V illustrates how the protocols we have reviewed to maximize performance. Table IV shows how helpers are switch from direct transmission to cooperative diversity when selected and grouped in the works we have reviewed. necessary. Given a method to optimize the helper set, there is an Unfortunately the design of a fair metric to compare dissim- extremely broad horizon of performance metrics. There are ilar transmission mechanisms is difficult. As cooperative diver- examples based on average information rate [55], transmission sity was born to fight fading, a first performance metric would time [46], covered distance [56], error probability [51], power likely be outage probability. Fading analysis would require consumption [57][54], power saving [40], battery lifespan a good knowledge about the geometry of the environment, [40], etc. Moreover, it is possible to design hybrid techniques which would still be very expensive in a general architecture. that allow simultaneous optimization over several of these A centralized topology with a complex base station (BS) is parameter domains. more favorable for outage analysis. A design with limited resources cannot afford a complete In any case, there is no reason for a single helper set. For outage analysis of a general scenario with aggressive channels. example, the 3-hop proposal with a transmitter helper group 1Wireless networks are traditionally classified as extensive or dense,the and a receiver helper group of figure 19 is used in [56], saving main difference being that in dense networks all nodes are directly reachable energy by shortening the ranges of the first and third hops (interference is the main constraint) while in extensive networks only the (which have a lower diversity and therefore trade less range closest neighbors are efficiently reachable (the distance in hops is the main constraint). The authors explain that, as an intermediate point, there is a improvement per power unit). regime where both constraints apply, whose optimal routing policy consists of dividing the network into directly-reachable cells, cooperating locally and In [58], it has been shown that the best policy for wireless performing multi-hop routing globally. GOMEZ-CUBA et al.: A SURVEY ON COOPERATIVE DIVERSITY FOR WIRELESS NETWORKS 11
TABLE IV TABLE VI TABLE OF STUDIED PROTOCOLS IN TERMS OF HELPER SET DESIGN. TABLE OF STUDIED PROTOCOLS IN TERMS OF NOTIFICATION AND AGREEMENT. Protocol Helper Set Design CMAC One random helper selected by contention Protocol Helper Notification FCMAC Random helper set selected by contention CMAC No ACK rDCF One optimal helper selected by the source FCMAC No ACK / Reception of NACK C-MAC Optimal CDMA helper set selected by the source rDCF DCF based triangular handshake OR One optimal helper selected by contention C-MAC Multiple explicit messages based on DCF PARS One optimal helper selected by contention OR DCF with SNR-proportional contention CD-MAC One optimal helper selected by the source PARS DCF with energy-proportional contention CTBTMA One optimal helper selected by contention CD-MAC Helper ID in data packets. C-DCF Phoenix One optimal helper selected by contention CTBTMA DCF with BT MAC and helper contention coopMAC-I One optimal helper selected by the source Phoenix NACK retransmission, DCF handshakes coopMAC-II One optimal helper selected by the source coopMAC-I DCF with ACK, helper indication on RTS c-coopMAC One optimal helper selected by the source coopMAC-II DCF without ACK, helper ID in data packets RcoopMAC Optimal helper set selected by the source c-coopMAC DCF with ACK, helper indication on RTS coopMAX Optimal helper set selected by the source RcoopMAC DCF without ACK, opportunistic selection One optimal helper selected by the source consider- coopMAX Helper announcement/allocation to/by BS MHA-CR ing multi-hop double relaying. MHA-CR Helper ID in data packets. C-DCF fairMAC One optimal helper selected by the source DCF with preACK when packets are accepted and fairMAC DC-MAC One optimal helper selected by contention jointACK when delivered DC-MAC DCF with contention TABLE V TABLE OF STUDIED PROTOCOLS IN TERMS OF COOPERATION DECISION. TABLE VII Protocol Cooperation Decision TABLE OF STUDIED PROTOCOLS IN TERMS OF TRANSMISSION METHODS. CMAC Reactive rtx after direct link failure Protocol Helper Notification FCMAC Reactive rtx after direct link failure Proactive selection at the source with heuristic credit CMAC Plain relaying rDCF system FCMAC Plain relaying C-MAC Iterative increment of cooperation gain rDCF Plain relaying OR Cooperation choice assumed C-MAC Simultaneous CDMA relaying PARS Proactive, source contention against relays OR Not specified CD-MAC Reactive backup upon direct link failure PARS Not specified CTBTMA Proactive relay announcement and contention CD-MAC OSTBC of source and relay Phoenix Reactive in case of NACK CTBTMA Plain relaying with AMC boosting coopMAC-I Min tx time, proactive source Phoenix CFNC and plain relaying coopMAC-II Min tx time, proactive source coopMAC-I Plain relaying with AMC boosting c-coopMAC Min tx time, proactive source coopMAC-II Plain relaying with AMC boosting RcoopMAC Min tx time, proactive source c-coopMAC Source combination and AMC boosting coopMAX Min tx time, proactive source RcoopMAC RDSTC relaying with AMC boosting MHA-CR Reactive backup upon direct link failure coopMAX RDSTC relaying with AMC boosting Min tx time, proactive source with pending ACK MHA-CR DSTC with multi-hop double relaying fairMAC counter fairMAC Plain relaying during fraction of time DC-MAC Proactive offer of helpers (hi packet) DC-MAC Plain relaying with AMC boosting
With limited knowledge, the MAC layer should take its chances and decide. For this purpose, performance metrics but it is necessary to decide if the transmitter, the helpers, or should be as generalist as possible to accept any available data. both send notifications to the receiver and which of them must There are examples that employ expected packet loss [52], receive the feedback from the receiver (if any). Additionally, number of routes [55], achievable rate [47][41], elapsed time it is necessary to decide which node is responsible of the final [46][8], and so on. In addition, the distinction in [5] between decisions in case of exception. There are examples of decisions proactive and reactive cooperation, depending on whether the taken at the source [46], the receiver [39], the helper [38], decision is made before or after transmission in the case of and some approaches provide different choices to the most primary link failure, should also be taken into account. informed peer [47]. 4) Cooperator Notification and Agreement: The helpers 5) Cooperative Transmission Design: The most versatile need to be notified of the cooperative aspects of the trans- PHY layers can be modified regarding number of relays, mission and the receiver should be aware of them as well. transmission time, code, modulation schema, etc. The MAC The notification is even more important when the cooperative layer should be capable of tuning these parameters to max- mechanism requires the exchange of initialization values. imize the chances of meeting the system requirements. Of Table VI shows the notification mechanisms of the protocols course, a one-size-fits-all solution, if this exists, would avoid in our review. There are difficulties when the helper set is the complexity of dynamic set-up switching. Nevertheless, chosen without information on helper willingness. Then, either for strong adaptiveness and exhaustive capacity exploitation, helper acknowledgement is required [42] or cooperation is some form of on-the-fly parameter design or selection is optimistically assumed [49]. If no ACK returns, some policy indispensable. Table VII summarizes the different approaches is necessary. The receiver may contribute to the agreement, to cooperative transmission in the protocols we have reviewed. 12 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, ACCEPTED FOR PUBLICATION
D. Comments on Security and Fairness In other words, the advantages of cooperative transmissions Zhu and Cao [38] anticipated security issues. Provided are only possible if the MAC layer is able to efficiently trace, that helper nodes are in fact other users, malicious relays classify and coordinate helpers at reasonable cost. For this might steal, modify or forge messages from fair users. Such purpose, most protocols rely on small control messages, such attacks are possible in all communication systems; hence the as the DCF of 802.11, or on a central controller, such as that use of cryptography, authentication and integrity checking is of 802.16. extended in them. The fact that cooperative diversity is a new There is no clear winner among the studied protocols, area where these attacks can be performed should not inval- because their features are better or worse depending on the idate the effectiveness of well-known protection techniques application domain. In this regard, there are five noteworthy already in use. On the other hand, new forms of malice might services that must be taken into account when selecting or de- arise in cooperative diversity services if greedy users distort signing a cooperative diversity MAC: neighborhood mapping, the coordination mechanism to their own benefit. helper set design, cooperation analysis and decision, cooper- Regarding fairness, adequate incentives are necessary for ator notification and agreement,andcooperative transmission the network users to share their resources willingly. For rapid design. deployment wireless networks cooperation is natural since all Some side effects that should be managed by the MAC layer nodes in the network are deployed under the same command. may arise from cooperative diversity: High energy expenditure The same is expected for wireless sensor networks since all of nodes in “good” positions (since many peers use them as sensor nodes are typically deployed by the same agent. relays), new security concerns, and interference redistribution In cellular data networks, mobile devices are independent. across cells. However, they are controlled by few service providers, which Most MAC protocols achieve their goals by relying on improve the efficiency of their networks according to cost in- previously existing protocols and tailoring them to cooperative centives. Those providers could offer cooperation incentives to diversity. Thus, it may be interesting to design new protocols end users by means of a well designed policy of requirements aiming primarily at the accomplishment of the theoretical and rewards. bounds rather than at retro-compatibility. Finally, in ad-hoc wireless data networks, the problem of Energy expenditure deserves more analysis as the results of cooperation incentives is hard to solve, since the users are not [54] show a trade-off between energy per bit and throughput grouped by providers or subject to service contracts. As a con- increase, whereas the results in [46], taking into account idle sequence, cooperation should be imposed by communication energy consumption, show potential power savings due to a standards with fairness-forcing mechanisms. reduction of idle waiting time. Integration with other network aspects, like routing, must E. On Interference in Cellular Networks be extended, as for example most models require source- destination signaling, preventing transmission towards nodes The simulation results in [4] and [46] for the coopMAC out of direct range that are cooperatively reachable. In general, protocols show that interfering signals are lower in cooperative routing and forwarding are heavily affected because node scenarios, which is apparently contradictory to the interference reachability depends on the cooperation environment. area extension in figure 1. This is due to the influence of the Another interesting field is the effect of network saturation time dimension, which must be taken into account. Although as most models assume that the incoming flow of packets is it is true that helper support does extend the interference area, unlimited and therefore any acceleration of the delivery pro- it also reduces the time that interference power is sustained. cess would increase throughput. Therefore, analysis of relaxed Also, the relay and the source do not transmit simultaneously, networks with plenty of resources should be investigated to so each influence area is active half the time. Therefore, there determine how cooperation affects jitter, delay, etc. is not an increase of interference in the interfered areas, but a redistribution. This reasoning could be extended to any other protocol with V. C ONCLUSIONS few modifications. It has to be considered that, if the nodes Cooperative diversity reaches the performance of multi- were allowed to transmit simultaneously, their interfered areas antennae systems at a lower cost, with smaller, single-antenna would become concatenated and, as a result, interfering power networked nodes. Based on recent analyses, the relation be- would effectively increase. Nevertheless, two transmissions tween transmission improvement and cooperation is evident have twice as much energy and consequently the comparison even for conveniently placed nodes. In their case, the gain would be unfair. If we perform an interference normalization comes from a faster medium release from inefficient neigh- on transmission power, using SINR metrics instead of plain bors. The overall increase of throughput has a cost in energy Watt power units, the effect of interference extension would that the peers need to consider when they decide to cooperate. be canceled by the power increase and the basic results of In this review we have discussed the information theoretical coopMAC would be extensible to any other MAC protocol. models that support cooperative diversity. However, these models are far from complete, and further work on them F. Conclussions and Open Issues is expected. 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Areas Commun., Degree (cum laude and Best PhD Thesis Award) in vol. 25, no. 2, pp. 340–354, 2007. Telecommunication Engineering from the University [47] F. Liu, T. Korakis, Z. Tao, and S. S. Panwar, “A MAC-PHY cross-layer of Vigo, Spain, in 2006 and 2009 respectively. He protocol for ad hoc wireless networks,” in Proc. WCNC, IEEE, 2008, has been a researcher with the Information Tech- pp. 1792–1797. nologies Group, University of Vigo, Spain until [48] F. Verde, T. Korakis, E. Erkip, and A. Scaglione, “On avoiding collisions 2009. Between 2008 and 2009 he was also R&D and promoting cooperation: Catching two birds with one stone,” in Manager at Optare Solutions, a Spanish telecommu- Proc. IEEE 9th Workshop on Signal Processing Advances in Wireless nications company. He currently holds an Ángeles Communications 2008, SPAWC 2008, pp. 431–435. Alvariño position, Xunta de Galicia, Spain. His interests include content [49] P. Liu, Y. Liu, T. Korakis, A. Scaglione, E. Erkip, and S. S. 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Panwar, “CoopMAX: A diant, Spain, as Research Director in Networks and cooperative MAC with randomized distributed space-time coding for an Applications. He leads the Information Technolo- IEEE 802.16 network,” in Proc. IEEE ICC 2009. gies Group, University of Vigo, Spain (http://www- [53] H. Adam, C. Bettstetter, and S. Senouci, “Multi-hop-aware cooperative gti.det.uvigo.es). relaying,” in Proc. IEEE 69th Vehicular Technology Conf. 2009, VTC He has published over fifty papers in international Spring 2009. journals, in the fields of telecommunications and [54] G. Bocherer and R. Mathar, “On the throughput/bit-cost tradeoff in computer science, and he has participated in several CSMA based cooperative networks,” in Proc. 2010 Intl. ITG Conf. on relevant national and international projects. He holds Source and Channel Coding (SCC), IEEE. a US patent.